H4 THE ORIGIN OF THE NERVOUS SYSTEM 



obscure. If the primary physiological relations in the 

 pattern of the organism are chemical or transportative 

 and excitation and transmission relations arise only 

 secondarily the localization of the nervous system with 

 respect to the gradients, its early development, and the 

 fact that it becomes the chief integrating organ all 

 remain remarkable and completely inexplicable facts of 

 ontogeny, but if it is true that organismic pattern 

 originates in excitation-transmission relations and that 

 the axial gradients are protoplasmic records of such 

 relations, all these facts appear in a new light. The 

 localization, the early differentiation, and the function 

 of the nervous system, which represents the specialized 

 mechanism of excitation and transmission in the organ- 

 ism, appear as the natural and necessary consequence 

 of the existence and physiological nature of the gradients. 

 Moreover, from this point of view nervous structure 

 appears as the primary or fundamental developmental 

 reaction of the more active body regions. The dis- 

 tinction between ectoderm and entoderm precedes its 

 appearance, but the entoderm represents physiologically 

 the lower levels of the chief axial gradient and the 

 nervous system is the first definite and definitive organ 

 to differentiate from the higher levels of the gradient or 

 gradients. 



The older conception of ontogeny as a process of 

 construction of a machine which, after construction is 

 completed, begins to function appears less and less satis- 

 factory as our knowledge advances. Living protoplasm 

 is functioning at all times and development is a process 

 of functional construction, that is, beginning with a 

 given structure and function, the continuance of func- 



